1. Loose-Ordering Consistency for Persistent Memory
Youyou Lu1, Jiwu Shu1,
Long Sun1, Onur Mutlu2
Hello, everyone. I am Youyou Lu from Tsinghua University.
Today, I will present our paper “Loose-Ordering Consistency for Persistent Memory”.
This is a joint work with Jiwu Shu, Long Sun from Tsinghua University and Onur Mutlu from Carnegie Mellon University.
1Tsinghua University
2Carnegie Mellon University

2. Summary
Problem: Strict write ordering required for storage consistency dramatically degrades performance in persistent memory
Our Goal: To keep the performance overhead low while maintaining the storage consistency
Key Idea: To Loosen the persistence ordering with hardware support
Eager commit: A commit protocol that eliminates the use of commit record, by reorganizing the memory log structure
Speculative persistence: Allows out-of-order persistence to persistent memory, but ensures in-order commit in programs, leveraging the tracking of transaction dependencies and the support of multi-versioning in the CPU cache
Results: Reduces average performance overhead of persistence ordering from 67% to 35%
In this paper, we work on the performance aspect of persistent memory while maintaining storage consistency.
The problem is that strict write ordering required for storage consistency dramatically degrades performance in persistent memory.
Our goal in this paper is to keep the performance overhead low while maintaining the storage consistency.
Our key idea is to loosen the persistence ordering with hardware support. We call our approach Loose-Ordering Consistency, the LOC.
We use two techniques in our approach.
The first is Eager Commit. Eager Commit is a new commit protocol. It eliminates the use of commit record. We achieve this by reorganizing the memory log structure.
The second is Speculative Persistence. Speculative Persistence allows out-of-order persistence to persistent memory, but ensures in-order commit in programs. We achieve this by leveraging the tracking of transaction dependencies and the support of multi-versioning in the CPU cache.
Our evaluation results show that our approach reduces average performance overhead of persistence ordering from 67% to 35%.

3. Outline Introduction and Background Existing Approaches
Our Approach: Loose-Ordering Consistency
Eager Commit
Speculative Persistence
Evaluation
Conclusion
In this talk, I will first give the introduction and the background.
And then, I will discuss the existing approaches.
After that, I will present our approach – Loose-Ordering Consistency, including the Eager Commit and Speculative Persistence techniques.
Finally, I will give the evaluation results and the conclusion.

4. Outline Introduction and Background Existing Approaches
Our Approach: Loose-Ordering Consistency
Eager Commit
Speculative Persistence
Evaluation
Conclusion
Let’s first come to the introduction and background.

5. Persistent Memory Persistent Memory Storage Consistency
Memory-level storage: Use non-volatile memory in main memory level to provide data persistence
Storage Consistency
Atomicity and Durability: Recoverable from unexpected failures
Boundary of volatility and persistence moved from Storage/Memory to Memory/Cache
LLC L2 L1
LLC L2 L1
Our paper talks about persistent memory.
Persistent memory is also called memory-level storage. It uses non-volatile memory in main memory level to provide data persistence.
The left figure shows the architecture of traditional storage systems, and the right figure shows the architecture of persistent memory.
In persistent memory, the memory provides data durability as well as the disk storage.
In storage systems, storage consistency is an important design issue. It provides atomicity and durability of data writes, so that the storage system is recoverable even after unexpected failures.
Persistent memory needs storage consistency, too. But the difference in ensuring storage consistency between disk-based storage systems and persistent memory is that: the boundary of volatility and persistence has moved from the interface of Storage and Memory to the interface of Memory and the CPU Cache.
Let me show a simple example.
In disk-based storage systems, …
In persistent memory, …
Memory
(DRAM)
Memory
(NVM)
Disk Storage
Disk Storage

6. Storage Consistency – Write-Ahead Logging(WAL)F I J’ J M’ O’ P’ J’ M’ M N O O’ P’
Step 1. Log Write
Step 2. Commit Record Write
Step 3. In-place Write
Step 4. Log Truncation
Intra-tx Ordering
Let’s use an example to look at the logical view of the Write-Ahead Logging (WAL). WAL is a heavily used protocol for storage consistency.
In the example, we want to update the green nodes in the tree and inserts a new node.
1) The first step of WAL is to write them to the log, (named log write).
2) After the data blocks have been written to the log area completely, a commit record is written to indicate the completeness of a tx. The commit record is used during recovery to determine the tx completeness.
3) Only after the commit record is written, those updated data blocks can be written to their home locations, as shown in Step 3.
4) Finally, the log can be truncated after the updated data blocks have been written completely to their home locations.
In the four steps, a transaction can send an acknowledge message to programs only after the commit record write. At that time, the durability and atomicity of a transaction are ensured. The atomicity is achieved using both the old version in their home locations and the new version in the log. The durability is achieved using the durable copy in the log.
And thus, Step 3 and 4 are asynchronously executed, and not in the critical path.
Instead, a tx can run only after its previous transaction completes. This waiting is the Inter-Tx Ordering in the figure.
Similarly, we call the waiting between the log write and the commit record write the Intra-Tx Ordering.
The two orderings are required for storage consistency, but lie in the critical path and hurt performance.
We aim to reduce or relax the two orderings in our paper.
Program Ack
Inter-tx Ordering
Ordering is required for storage consistency.

7. High Overhead for Ordering in PM
Persistence ordering
Force writes from volatile CPU cache to Persistent Memory L1 L2
Volatile
LLC
Memory
(NVM)
Persistent
High overhead for persistence ordering
The boundary between volatility and persistence lies between the H/W controlled cache and the persistent memory
Costly software flushes (clflush) and waits (fence)
Existing systems reorder writes at multiple levels, especially in the CPU and cache hierarchy
The ordering we discussed specifically is referred to Persistence Ordering. Persistent ordering is used to write sequence keeping for writes from volatile media to persistent media. In persistent memory, to keep persistence ordering, it forces writes from the volatile CPU cache to the persistent memory.
As shown in the figure, the CPU cache is volatile, and the memory is persistent. Persistence ordering is ensured by flushing from CPU cache to memory.
Different from disk-based storage systems, persistence ordering causes even higher overhead in persistent memory.
There are two reasons.
The first is that The boundary between volatility and persistence lies between the H/W controlled cache and the persistent memory. And thus, the software explicitly flushes data blocks in the programs and waits for the completion.
The second is that Existing systems reorder writes at multiple levels, especially in the CPU and cache hierarchy. It has to iterate each cache level.

8. Outline Introduction and Background Existing Approaches
Our Approach: Loose-Ordering Consistency
Eager Commit
Speculative Persistence
Evaluation
Conclusion
To address this problem, before introducing our LOC approach, I will first discuss existing approaches.

9. Existing Approaches Making the CPU cache non-volatile
Reduce the time gap between volatility and persistence by employing a non-volatile cache
Is complementary to our LOC approach
Allowing asynchronous commit of transactions
Allow the execution of a later transaction without waiting for the persistence of previous transactions
Allow execution reordering, but no persistence reordering 3 2 1
Existing approaches can be divided into two categories in general.
The first category is making the CPU cache non-volatile. This category of approaches aim to reduce the time gap between volatility and persistence by employing a non-volatile cache.
The examples shows the execution of four transactions (T1 to T4). The figure shows one approach that uses non-volatile last level cache (LLC). Each transaction is committed only when their data blocks are written to the non-volatile cache.
Fortunately, this category of approaches is complementary to our LOC approach.
The second category of approaches is Allowing asynchronous commit of transactions. Approaches of this category Allow the execution of a later transaction without waiting for the persistence of previous transactions. The hardware keeps the ordering, and the software queries asynchronously.
As shown in the right figure, T1 is written to the cache. But when the program needs to write T2, T1 should be flushed to PM, because T1 has overlapping data block write (block A) with T2. After T2 is written to the CPU cache, T3 can also be written to the CPU cache without forcing T2 to PM. At this time, the program begin to execute T4, while the actual committed tx is T1. In this way, the commit is asynchronous.
However, the persistence ordering is still kept. Even though the execution order can be changed, the persistence order can not.
T1: A, B, C, D
T2: A, F
T3: B, C, E
T4: D, E, F, G
LLC L2 L1 3 1 2 4
LLC L2 L1 3 1 2 4 1
LLC
Memory
(NVM)
Memory
(NVM)

10. Our Solution: Key Ideas4 3 2 1
LLC L2 L1
Loose-Ordering Consistency (LOC)
Allow persistence reordering 3 1 2 4 3 1
Memory
(NVM)
Eager Commit
Remove the intra-tx ordering
Delay the completeness check till recovery phase
Reorganize the memory log structure
Speculative Persistence
Relax the inter-tx ordering
Speculatively persist transactions but make the commit order visible to programs in the program order
Use cache versioning and Tx dependency tracking
Our approach is to allow persistence reordering. And our approach is called Loose-Ordering Consistency, simply the LOC.
As shown in the figure, LOC allows txs to be written to the CPU cache without forcing them to the PM. Also, data blocks in txs can be written to PM in any order.
In order to achieve this, we have two techniques: Eager Commit and Speculative Persistence.
Eager Commit aims to remove the intra-tx ordering. It delays the completeness check till recovery phase. This is achieved by reorganizing the memory log structure.
Speculative Persistence aims to relax the inter-tx ordering. It speculatively persists transactions but make the commit order visible to programs in the program order. This is achieved by using cache versioning and Tx dependency tracking.
The two techniques are to be discussed next.

11. Outline Introduction and Background Existing Approaches
Our Approach: Loose-Ordering Consistency
Eager Commit
Speculative Persistence
Evaluation
Conclusion
Now, let us come to part of our approach – Loose-Ordering Consistency.
I will discuss the two techniques Eager Commit and Speculative Persistence of LOC.

12. LOC Key Idea 1 – Eager Commit
Step 1. Log Write
Step 2. Commit Record Write
Step 3. In-place Write
Step 4. Log Truncation
Intra-tx Ordering
Program Ack
Inter-tx Ordering
Goal: Remove the intra-tx ordering
Eager Commit: A new commit protocol without commit records
Before discussing the Eager Commit, let us first review the four steps in a transaction. In the four steps, the first two steps are in critical path, and there are two orderings, intra-tx and inter-tx ordering.
The goal of Eager Commit is to remove the intra-tx ordering.
Eager Commit is a new commit protocol without commit records.

13. Eager Commit Commit Protocol Eager Commit
Commit record: Check the completeness of log writes
Eager Commit
Reorganize the memory log structure for delayed check
Remove the commit record and the intra-tx ordering
Use count-based commit protocol: <TxID, TxCnt>
In order to remove the use of commit records, we need to understand why traditional WAL uses commit records.
In commit protocols, a commit record is written after the writes have been completely written to the log area. On recovery, the existence of a commit record is used to determine whether the transaction has been committed. If there is a commit record, the tx is committed. Otherwise, it is not committed. Therefore, during normal runs, the commit protocol makes sure all log writes are complete and then writes a commit record.
Eager Commit aims to remove the commit records. This can be achieved by reorganizing the memory log structure and thus delaying the completeness check on recovery.
Figure: in the memory log area, we divide the memory into block groups. Each block group consists of eight blocks, in which seven are used for data blocks and the other is used for metadata block.
The metadata block keeps the metadata information for the seven data blocks.
In the metadata, we use TxID and TxCnt for a count-based commit protocol.

14. Eager Commit No commit record. Intra-tx ordering eliminated.
Count-based commit protocol
During normal run,
Tag each block with TxID
Set only one TxCnt to the total # of blocks in the tx, and others to ‘0’
During recovery,
Recorded TxCnt: Find the non-zero TxCnt for each tx TxID
Counted TxCnt: Count the tot. # of blocks in the tx
If the two TxCnts match (Recorded = Counted), committed; otherwise, not-committed
This slide explains the count-based commit protocol.
In the count-based commit protocol, during normal run, each data block is tagged with a TxID in the metadata.
For each tx, only one block has TxCnt set to total # of blocks in the tx, and others are set to‘0’.
The non-zero TxCnt can be used in the recovery phase for completeness check.
During recovery, it first searches the non-zero TxCnt in each tx. The TxCnt is the recorded TxCnt.
And then, it count the total number of blocks in a tx. This TxCnt is the counted TxCnt.
If the two values match, which means recorded TxCnt equals Counted TxCnt, the tx is committed. Otherwise, the tx is not committed.
In this way, we avoid the use of commit records, and thus the intra-tx ordering.
No commit record. Intra-tx ordering eliminated.

15. LOC Key Idea 2 – Speculative Persistence
Step 1. Log Write
Step 2. Commit Record Write
Step 3. In-place Write
Step 4. Log Truncation
Intra-tx Ordering
Program Ack
Inter-tx Ordering
Goal: relax the inter-tx ordering
Speculative Persistence
Out-of-order persistence: To relax the inter-tx ordering to allow persistence reordering
In-order commit: To make the tx commits visible to programs (program ack) in the program order
Let’s come to the second technique: Speculative Persistence.
As discussed just now, the Eager Commit technique has removed the use of commit records and thus the intra-tx ordering.
The second technique, Speculative Persistence, work on the inter-tx ordering as well as the program acknowledgement.
The goal of Speculative Persistence is to relax the inter-tx ordering.
Speculative persistence allows out-of-order persistence. This relaxes the inter-tx ordering to allow persistence reordering.
But it ensures in-order commit. In-order commit means to make the tx commits visible to programs (program ack) in the program order.

16. Speculative Persistence
T1: (A, B, C, D) -> T2: (A, F) -> T3: (B, C, E) -> T4: (D, E, F, G)
Strict Ordering
volatile CPU cache A A A B B B C C C D D D A A A F F F B B B C C C E E E D D D E E E F F F G G G
persistent memory A B C D A F B C E D E F G
Loose Ordering
volatile CPU cache A A B B C C D D A A A F F B B B C C C E E D D D E E E F F F G G G
I will use an example to show how Speculative Persistence works. In the example, we compare the strict ordering, the legacy way, and the loose ordering, which uses the Speculative Persistence technique.
In each of the two figures, the top rectangles are volatile CPU cache blocks, and the bottom rectangles are blocks in persistent memory.
The purple line is the program ack. Before the line, all transactions are committed. After the line, all transactions are not committed.
In Strict Ordering,…
One by one
In Loose Ordering, transactions runs without forcing blocks to persistent memory.
Speculative Persistence allows persistence of data blocks out-of-order. For example, T3 can persist data blocks before T2. …
The program ack moves if the tx is committed.
Speculative Persistence commits in order. For example, when E is persisted, T3 is committed. But since T2 is not, the program ack does not move.
Only after F is persisted, the program ack moves two txs forward.
Also, data blocks can be merged if two transactions have overlapping writes. This is the write coalescing.
Thus, Speculative Persistence relaxes the inter-tx ordering, and enables write coalescing.
persistent memory A B C D E F G
Inter-tx ordering relaxed. Write coalescing enabled.

17. Speculative Persistence
Speculative Persistence enables write coalescing for overlapping writes between transactions.
But there are two problems raised by write coalescing of overlapping writes:
How to recover a committed Tx which has overlapping writes with a succeeding aborted Tx?
Overlapping data blocks have been overwritten
Multiple Versions in the CPU Cache
How to determine the commit status using the count-based commit protocol of a Tx that has overlapping writes with succeeding Txs?
Recorded TxCnt != Counted TxCnt
Commit Dependencies between Transactions
Tx Dependency Pair: <Tp, Tq, n>
While Speculative Persistence enables write coalescing for overlapping writes between transactions, there are two problems raised by write coalescing of overlapping writes.
The first is How to recover a committed Tx which has overlapping writes with a succeeding aborted Tx?
A committed transaction may have data blocks overwritten by a later aborted tx.
The second is How to determine the commit status using the count-based commit protocol of a Tx that has overlapping writes with succeeding Txs?
In this case, the counted TxCnt is smaller than the recorded TxCnt. The tx can not be determined to be committed using the count-based commit protocol.
To the first question, Speculative Persistence supports multiple versions in the CPU cache. The committed transaction can have its overwritten data blocks recovered.
To the second question, Speculative Persistence keeps commit dependencies between txs using Tx Dependency Pairs <Tp, Tq, n>. The pair means Tq overwirtes n pages in Tp. Thus, n is added to the counted TxCnt to compare the counted and recorded TxCnt.
Details of the two can be found in the paper.
See the paper for more details.

18. Recovery Recovery is made by scanning the memory log.
More details in the paper.
As for recovery, it is made by scanning the memory log.
We discuss how to handle transaction recovery in our paper. Due to lack of time, I do not discuss it in this talk, but I would be happy to answer any questions.

19. Outline Introduction and Background Existing Approaches
Our Approach: Loose-Ordering Consistency
Eager Commit
Speculative Persistence
Evaluation
Conclusion
Next, I will show some evaluation results.

20. Experimental Setup GEM5 simulator Simulator configuration Workloads
Timing Simple CPU: 1GHz
Ruby memory system
Simulator configuration
L1: 32KB, 2-way, 64B block size, latency=1cycle
L2: 256KB, 8-way, 64B block size, latency=8cycles
LLC: 1MB, 16-way, 64B block size, latency=21cycles
Memory: 8 banks, latency=168cycles
Workloads
B+ Tree, Hash, RBTree, SPS, SDG, SQLite
In the evaluation, we use the full system simulator, the GEM5 simulator.
We configure the simulator use parameters shown in the slide.
And we evaluate different workloads, including commonly used data structures for storage, such as B+ tree, database workload, SQLite, and others.

21. Overall Performance
We first show the overall performance. All the performance has been normalized to the performance without persistence ordering.
The gap between the bar to the top line (value ‘1’) is the performance overhead of persistent ordering.
So, in the y-xis of this figure, lower bars means higher performance overhead of persistence ordering.
In the figure, H-WAL is WAL implemented in the hardware, and the LOC-WAL is the H-WAL using our approach: Loose-Ordering Consistency.
By comparing the H-WAL, we can see LOC significantly improves performance of WAL. It reduces average performance overhead of persistence ordering from 67% to 35%.
Also in the figure, Kiln is a newly proposed approach to use non-volatile LLC in MIRCO We incorporate LOC with Kiln and call it LOC-Kiln.
The results show LOC and Kiln can be combined favorably.
We conclude that LOC effectively mitigates performance degradation from persistence ordering.
LOC significantly improves performance of WAL: Reduces average performance overhead of persistence ordering from 67% to 35%.
LOC and Kiln can be combined favorably.
LOC effectively mitigates performance degradation from persistence ordering.

22. Effect of Eager Commit
We study the effect of the Eager Commit technique by comparing the H-WAL and EC-WAL. EC-WAL is the protocol that uses only Eager Commit in H-WAL.
It shows Eager Commit outperforms H-WAL by 6.4% on average due to the elimination of intra-tx ordering.
Eager Commit outperforms H-WAL by 6.4% on average due to the elimination of intra-tx ordering.

23. Effect of Speculative Persistence
We also study the effect of Speculative Persistence.
From the figure, we can see the performance increases as the speculative degree increases.
Speculative Degree is the maximum number of transactions that are allowed to be persisted at one time.
Speculative Persistence improves the normalized transaction throughput from (SD=1) to (SD=32) with a 95.5% improvement.
The larger the speculation degrees, the larger the performance benefits.
Speculative Persistence improves the normalized transaction throughput from (SD=1) to (SD=32) with a 95.5% improvement.

24. Outline Introduction and Background Existing Approaches
Our Approach: Loose-Ordering Consistency
Eager Commit
Speculative Persistence
Evaluation
Conclusion
Finally, let me conclude the talk.

25. Conclusion
Problem: Strict write ordering required for storage consistency dramatically degrades performance in persistent memory
Our Goal: To keep the performance overhead low while maintaining the storage consistency
Key Idea: To Loosen the persistence ordering with hardware support
Eager commit: A commit protocol that eliminates the use of commit record, by reorganizing the memory log structure
Speculative persistence: Allows out-of-order persistence to persistent memory, but ensures in-order commit in programs, leveraging the tracking of transaction dependencies and the support of multi-versioning in the CPU cache
Results: Reduces average performance overhead of persistence ordering from 67% to 35%

26. Loose-Ordering Consistency for Persistent Memory
Youyou Lu1, Jiwu Shu1,
Long Sun1, Onur Mutlu2
1Tsinghua University
2Carnegie Mellon University